This section provides background information related to the present disclosure which is not necessarily prior art.
A secondary lithium-ion battery is a rechargeable power source that can be implemented into a wide variety of stationary and portable applications. The structure and electrochemical reaction mechanism of this type of battery provide several desirable characteristics, including a relatively high energy density, a relatively low internal resistance, a general nonappearance of any memory effect as compared to other types of rechargeable batteries (e.g., a nickel-cadmium battery), and a low self-discharge rate. These characteristics make the lithium-ion battery a preferred mobile power source for portable consumer electronics such as laptop computers and cell phones. Larger-scale versions that interact with a multitude of interconnected systems are employed in the automotive industry to improve vehicle fuel efficiency and reduce atmospheric pollution. The powertrains of hybrid electric vehicles (HEV) and extended range electric vehicles (EREV), for example, can rely on the cooperative effort of multiple lithium-ion batteries and a hydrocarbon-fueled internal combustion engine to generate power for vehicle operation.
A lithium-ion battery generally contains one or more electrochemical battery cells that each include a negative electrode, a positive electrode, and a porous separator sandwiched between confronting inner face surfaces of the electrodes. Each of these battery components is wetted with a liquid electrolyte solution that can communicate lithium ions. The negative and positive electrodes are formed of different materials that can intercalate and de-intercalate lithium ions and, when connected, establish an electrochemical potential difference. An interruptible external circuit connects the negative electrode and the positive electrode to provide an electrical current path around the separator to electrochemically balance the migration of lithium ions through the separator between the electrodes. Metallic current collectors intimately associated with each electrode supply and distribute electrons to and from the external circuit depending on the operating state of the electrochemical battery cell. The external circuit can be coupled to an electrical load (during discharge) or an applied voltage from an external power source (during charging) through conventional electronic connectors and related circuitry.
The porous separator includes opposed major face surfaces that intimately contact the confronting inner face surfaces of the electrodes. Functions of the separator include providing a porous and electrically insulative mechanical support barrier between the negative and positive electrodes to prevent a short-circuit. Conventionally, the porous separator has been composed of a polyolefin such as polyethylene and/or polypropylene. A number of fabrication methods exist for making a polyolefin separator with its intended porosity. For example, the separator can be formed by a dry technique in which a polyolefin polymer is melted, extruded into a film, annealed, and then uniaxially stretched. The separator can also be formed by a wet technique in which a polyolefin polymer is mixed with a hydrocarbon or other low-molecular weight liquid substance. The mixture is then heated, melted, extruded into a sheet, and biaxially stretched. Afterwards, the hydrocarbon or other low-molecular weight liquid substance is extracted.
A polyolefin separator, however, can be susceptible to certain performance declines when heated excessively. Exposure of the electrochemical battery cell to temperatures of 80° C. and above can cause the polyolefin separator to shrink, soften, and even melt. Such high temperatures can be attributed to charging-phase heat generation, ambient atmospheric temperature, or some other source, Physical distortion of the polyolefin separator can ultimately permit direct electrical contact between the negative and positive electrodes and cause the electrochemical cell to short-circuit. Battery thermal runaway is also a possibility if the electrodes come into direct electrical contact with one another. The inability of a polyolefin separator to maintain thermal stability at temperatures exceeding 80° C. for prolonged periods can be a limitation in certain lithium-ion battery applications.